We used to just Alt + F4 to close the game if we didn't get their car and reply the boss races, especially for a few cars.

The heat level goes only upto level 2 at blacklist 15 and it starts to increases one level after defeating a few main guys.

Interestingly there is a challenger series race where you have to evade the police with heat level 7! They are using all SUVs in that pursuit and gets very challenging.

!LUV
!PIZZA